A Bullet for the General

Quién sabe? is a movie starring Gian Maria Volontè, Klaus Kinski, and Martine Beswick. A band of Mexican gun-runners employed by a revolutionary general accept an American into their gang, unaware of his intentions.
El Chuncho's bandits rob arms from a train, intending to sell the weapons to Elias' revolutionaries. They are helped by one of the passengers, Bill Tate, and allow him to join them, unware of his true intentions.